1. Field of the Invention
This invention relates to a thermoformable thermoplastic composite sheet material which is strong and light and which is capable of being thermally deformed and stretched to any desired shape without buckling, rippling or tearing of the composite material.
2. Description of Prior Art
It is often desirable to form a sheet of plastic material into a complex shape without any rippling, buckling, or tearing of the material. It is also desirable to make such sheet as light (from a weight standpoint), thin and strong as possible. It is also desirable to form a sheet with the above characteristics into the complex shape in a relatively simple process so that the costs of providing the complex shape in the sheet are low and so that the yield of the final product is high.
A sheet with the properties discussed may have a wide variety of different uses. For example, it may be shaped to form a container, or the sides of a container such as used in air cargo. Other uses may be to form components or portions of a boat or vehicle. Although the examples have been limited to uses in transportation, this is only one of a number of different fields or areas in which such material can be used.
Thermoplastic materials have been used to provide sheets with the properties discussed above. To obtain a desired shape, a mold is provided with the desired shape. The thermoplastic material is then formed into the desired shape by applying heat and pressure to the thermoplastic material to move the material into the mold and to have the material adopt the configuration of the mold. Suitable materials for forming into complex shapes may be any thermoplastic polymer including acrylics, ABS, polyolefins, nylon and polycarbonates and the engineering thermoplastic such as polysulfones and PBT (polybutyleneteraphthalate, and the new thermoplastic resins such as PEEK (polyetheretherketone), PEKK (polyetherketoneketone) etc.
Thermoplastic materials are advantageous because they can be reshaped if it is desired to change the configuration somewhat after the material has been initially shaped through the application of heat and pressure. However, thermoplastic materials are distinctly disadvantageous in that they have to be provided with a considerable thickness in order to provide the necessary rigidity. For example, thicknesses of 0.120" to 0.200" may be required. Unfortunately, such thick materials are heavy and weight is a disadvantage in transportation. Furthermore, the thermoplastic materials are not as durable and resistant to breaking as would otherwise be required.
Thermosetting materials that contain reinforcing fibers have been used to provide complex shapes. An advantage of a thermosetting material is that they can be made quite thin to obtain the desired shape. For example, the thickness of the thermosetting material may be in the order 0.125" and be as stiff as material 0.200". One problem with shaping thermosetting materials into complex shapes is that the thermosetting materials have to be shaped properly the first time. The reason is that the thermosetting materials cannot be reshaped after they have been heated to a temperature for initially shaping the material. This problem has limited the use of thermosetting material to provide large complex shapes. "Another problem often is that the weight of the thermosetting material tends to be heavy and irregular even though the part is thin."
Fiberglass is a thermosetting material that uses styrene based polybutylene resin glass as a reinforcement. "Open Molding" is used to fabricate large "fiberglass" parts for businesses traditionally needing modest production volumes. Part costs for such open molding are growing rapidly because throughput is slow, labor content is high and open molding is unfriendly to man and his environment. One reason fabricators keep using "fiberglass" is familiarity. It was developed in the late 40's. Raw materials are inexpensive and make good structural parts. Tooling is cheap and is made rapidly. Another reason for the use of "fiberglass" is the lack of viable alternatives. Unfortunately, rising part costs, worker's health and environmental issues are fundamental problems with "fiberglass". These problems can not easily be resolved.
Matched metal mold technology which uses similar styrene containing materials is available. It uses heated molds to rapidly cure material. This reduces health and environmental issues and eliminates hazardous waste. However, it is too expensive to be competitive at production volumes needed for open molded parts.
The fiberglass technology is based on open air curing (polymerizing) of styrene. Parts are made by painting a styrene containing polyester resin onto glass fabric. This fabric/resin layer is built up on a mold surface to the needed thickness. The parts harden within 4 hours.
Part fabrication using fiberglass technology is labor intensive and costs are rising. The labor cost can be and greater of the cost of goods. The fiberglass process is unfriendly to workers and their environment. Styrene is volatile and the chemically reactive material mix effects workers as follows:
1. Workers Health
Worker's Compensation Insurance rates are higher for workers, who inhale styrene daily. It is known that aromatic chemicals like styrene damage liver and kidney. Styrene is a possible carcinogen and is on the IRAC watch list.
2. Air Pollution
Styrene pollutes the air. Emissions are regulated by permit. Company growth can be stopped if all permit capacity for a given plant is in use.
3. Hazardous Waste Disposal
The chemically reactive mix forms a hazardous waste. Disposal fees are rapidly increasing as dump sites get scarce.
Thermoforming parts from glass or other fiber reinforced thermoplastic sheet would be an alternative to "fiberglass technology", but this new concept has unresolved technical problems. The thermoforming aspect is an old fabrication technology which started in the 1950's. This technology is used to form parts in modest production volumes. It offers low labor costs, fast forming cycles and low cost tooling. It uses vacuum to form parts from heat softened thermoplastic sheet that is clamped in a frame on top of the mold. Plastics include polyolefines, acrylics, ABS, polycarbonate nylons, etc. The problem is with materials. The fiber reinforced thermoplastic sheets available today, are not adequate for the job of thermoforming acceptable parts.